1. Field of the Invention
The present invention relates to a plasma etching method used for the manufacturing of semiconductor devices and so on, and more particularly to an electronic cyclotron resonance (ECR) etching technique or a microwave plasma etching technique (hereinafter these methods are collectively referred to as the "plasma etching method") which may be utilized in the field of the formation of gate structures and so on which needs to form a polysilicon film on a thin SiO.sub.2 film of about 200 Angstrom or less with a good dimensional accuracy. More specifically, the present invention relates to a method for etching polysilicon where the underlying structures have vertical steps. Incidentally, it is assumed that the polysilicon includes both undoped polysilicon and doped polysilicon.
2. Description of the Related Art
Conventionally, in plasma etching method to form a polysilicon film on a silicon substrate with a good dimensional accuracy, FREON 113 (C.sub.2 Cl.sub.3 F.sub.3) and a mixed gas containing this and a slight amount of sulfur hexafluoride (SF.sub.6) are used as a process gas. In recent years, C.sub.2 Cl.sub.3 F.sub.3 is believed to cause the destruction of the ozone layer, so that its use tends to be prohibited. As a material substituting for C.sub.2 Cl.sub.3 F.sub.3, chlorine or a process gas containing chlorine and a slight amount, i.e., 10 vol. % or less of SF.sub.6 is used.
However, the above-mentioned etching methods have the following problems:
The method using C.sub.2 Cl.sub.3 F.sub.3 has an environmental problem of damaging the ozone layer in the stratosphere as described above. Further, since the etchrate selectivity (i.e. the etchrate of polysilicon: etchrate of SiO.sub.2) is only about 20:1, the underlying SiO.sub.2 film may be damaged or completely removed.
In the etching method using chlorine, when over-etching is performed, the profile shape of a polysilicon film having mask 12 thereon is gradually tapered with a re-entrant profile, as shown in FIG. 1, which results in deterioration of dimensional accuracy. This re-entrant profile makes observation and evaluation using a scanning electron microscope (SEM) difficult. Further, undercut of the polysilicon film 11 may result in a notch 15, as shown in FIG. 2, which sometimes results in a crucial defect. This notch is believed to occur due to chlorine ions and chlorine radicals attaching to the polysilicon side wall and etching the polysilicon.
On the other hand, the etching method using a process gas containing chlorine and SF.sub.6 may give rise to undercut due to isotropic etching. When a photoresist mask is used as a mask, anisotropic etching is somehow achieved by a side wall protection of the photoresist if SF.sub.6 flow rate is reduced to about 5% or less of the chlorine flow rate, with the result that a substantially vertical polysilicon film side wall is formed. However, when a SiO.sub.2 mask is used as a mask, a side wall protecting material is not present so that undercut advances to a remarkable extent. This method, thus, cannot be applied to a polysilicon film with a line width of about 0.5 micron or less.